1. Field of the Invention
This invention relates to collection of suspended particles and more in particular to a cleaning device of an electrophotographic copying machine for cleaning the surface of a photosensitive member by removing and collecting the toner particles remaining thereon. More specifically, the present invention relates to an apparatus for collecting suspended toner particles with high efficiencies.
2. Description of the Prior Art
In a transfer type electrophotographic copying machine, a toner image is formed on the surface of a photosensitive member by going through the steps of an electrophotographic image forming process and a copy paper is brought into contact with the surface of the photosensitive member to transfer the toner image onto the copy paper. After the transfer step, there remains toner particles on the surface of the photosensitive member which have not been transferred. As a preparation for the next cycle of copying operation, these residual toner particles must be removed from the surface of the photosensitive member.
Customarily, use has been made of several cleaning techniques such as blade cleaning, fur-brush cleaning, web cleaning, etc. In the fur-brush cleaning technique, the toner particles remaining on the surface of the photosensitive member are scraped off by the fur brush and the thus scraped-off toner particles are then removed from the fur brush to be suspended to an air flow which is led into a filter chamber for collecting the suspended toner particles by a filter. Such a conventional fur-brush cleaning technique is impracticable in many respects, however. For example, filters must be replaced quite often since a relatively large amount of toner particles is collected. If it is desired to use a filter for a longer period of time, there must be provided an air flow having a sufficient momentum to overcome the pressure drop caused by plugging of the filter interstices by the collected toner particles. This then requires to increase the rotational velocity of the fur brush or to provide a suction device at the downstream end.
One prior art approach to cope with the above-described problems of the fur-brush cleaning technique is shown in FIGS. 1 and 2. As shown, a photosensitive member 1 moves in the direction indicated by the arrow A and toner particles 2 are carried on the surface of the photosensitive member 1. A fur brush 3 is driven to rotate in the direction indicated by the arrow B and therefore the toner particles 2 are scraped off by the fur brush 3. An obstruction plate 4 is provided in engagement with the fur brush 3. Thus, as the fur brush 3 rotates, the toner particles are removed from the fur brush 3 by the obstruction plate 4 and they become suspended in an air flow caused by the rotation of the fur brush 3.
Then the air flow with the suspension of toner particles is directed to a passage defined between a roller electrode 5 and a counter electrode 6. A high voltage V of the negative polarity is applied to the roller electrode 5 and the counter electrode 6 is grounded so that an electric field is formed therebetween. Toner particles in the air flow still retain charges and, in this case, it is assumed that the toner particles are positively charged. Thus, the toner particles in the air flow are attracted toward the roller electrode 5 and collected on the peripheral surface thereof. The roller 5 is driven to rotate in the direction indicated by the arrow C and a flexible blade 7 is provided in contact with the peripheral surface of the roller 5. Thus, as the roller 5 rotates, the toner particles now collected on the roller 5 are scraped off by the blade 7 and transported into a toner collection chamber 8. The toner particles collected in the chamber 8 may be further transported to a developing station (not shown) of a copying machine for reuse. On the other hand, the air flow from which the suspended toner particles have been removed is led into a filter chamber 9 and discharged out into the exterior through a filter 11.
FIG. 2 is a vertical cross-section of the roller 5 and its surrounding elements of the device shown in FIG. 1. The roller 5 is rotatably journaled to a support member 11 of an electrically insulating material. The support member 11 also constitutes sidewalls of the air flow passage. In the absence of toner particles, even if a high voltage is applied between the electrodes 5 and 6, a breakdown through the support member 11 may be prevented from occurring by selecting an appropriate material for the support member 11. However, when an air flow with suspension of toner particles is pressed through the passage, the toner particles are mostly collected on the roller 5 and, at the same time, some of the toner particles are collected on the support member 11 as well as the electrode 6 to form a thin toner layer thereon. For the sake of simplicity, only the toner layer 12 formed on the support member 11 is shown in FIG. 2. Repulsion between likely charged particles and fluiddynamic turbulence would cause some of the suspended toner particles to be deposited on the support member 11.
Since a high voltage is applied between the electrodes 5 and 6 and the toner layer 12 may be formed to bridge these electrodes, a creeping discharge could occur along the toner layer 12. Once occurred, the high voltage can no longer be sustained between the electrodes 5 and 6 because a leakage path is created along the toner layer 12. Studies have shown that the measured resistance between the electrodes 5 and 6 prior to the occurrence of creeping discharge was approximately 100 megohms for a particular setup with the application of 5 kV high voltage and the resistance dropped to about 1 kilohms after the occurrence of creeping discharge. Therefore, the applied voltage must be lowered after the occurrence of creeping discharge, which necessarily reduces the collection efficiency of the toner particles. Moreover, creeping discharge could cause damages not only to the electrodes and the support member, but also to the voltage supply and its control circuit. Particularly in the case of a copying machine, it is hazardous for the machine operator.